--- a/dom/events/ContentEventHandler.cpp
+++ b/dom/events/ContentEventHandler.cpp
@@ -3,32 +3,34 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "ContentEventHandler.h"
 #include "mozilla/IMEStateManager.h"
 #include "mozilla/TextEvents.h"
 #include "mozilla/dom/Element.h"
+#include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/dom/Selection.h"
 #include "nsCaret.h"
 #include "nsCOMPtr.h"
 #include "nsContentUtils.h"
 #include "nsCopySupport.h"
 #include "nsFocusManager.h"
 #include "nsFontMetrics.h"
 #include "nsFrameSelection.h"
 #include "nsIContentIterator.h"
 #include "nsIPresShell.h"
 #include "nsISelection.h"
 #include "nsISelectionController.h"
 #include "nsIFrame.h"
 #include "nsIObjectFrame.h"
 #include "nsLayoutUtils.h"
 #include "nsPresContext.h"
+#include "nsQueryObject.h"
 #include "nsRange.h"
 #include "nsTextFragment.h"
 #include "nsTextFrame.h"
 #include "nsView.h"
 
 #include <algorithm>
 
 namespace mozilla {
@@ -491,47 +493,53 @@ ContentEventHandler::ShouldBreakLineBefo
   if (aContent->IsHTMLElement(nsGkAtoms::br)) {
     return IsContentBR(aContent);
   }
 
   // Note that ideally, we should refer the style of the primary frame of
   // aContent for deciding if it's an inline.  However, it's difficult
   // IMEContentObserver to notify IME of text change caused by style change.
   // Therefore, currently, we should check only from the tag for now.
-  // TODO: Check if the element is an unknown HTML element.
-  return !aContent->IsAnyOfHTMLElements(nsGkAtoms::a,
-                                        nsGkAtoms::abbr,
-                                        nsGkAtoms::acronym,
-                                        nsGkAtoms::b,
-                                        nsGkAtoms::bdi,
-                                        nsGkAtoms::bdo,
-                                        nsGkAtoms::big,
-                                        nsGkAtoms::cite,
-                                        nsGkAtoms::code,
-                                        nsGkAtoms::data,
-                                        nsGkAtoms::del,
-                                        nsGkAtoms::dfn,
-                                        nsGkAtoms::em,
-                                        nsGkAtoms::font,
-                                        nsGkAtoms::i,
-                                        nsGkAtoms::ins,
-                                        nsGkAtoms::kbd,
-                                        nsGkAtoms::mark,
-                                        nsGkAtoms::s,
-                                        nsGkAtoms::samp,
-                                        nsGkAtoms::small,
-                                        nsGkAtoms::span,
-                                        nsGkAtoms::strike,
-                                        nsGkAtoms::strong,
-                                        nsGkAtoms::sub,
-                                        nsGkAtoms::sup,
-                                        nsGkAtoms::time,
-                                        nsGkAtoms::tt,
-                                        nsGkAtoms::u,
-                                        nsGkAtoms::var);
+  if (aContent->IsAnyOfHTMLElements(nsGkAtoms::a,
+                                    nsGkAtoms::abbr,
+                                    nsGkAtoms::acronym,
+                                    nsGkAtoms::b,
+                                    nsGkAtoms::bdi,
+                                    nsGkAtoms::bdo,
+                                    nsGkAtoms::big,
+                                    nsGkAtoms::cite,
+                                    nsGkAtoms::code,
+                                    nsGkAtoms::data,
+                                    nsGkAtoms::del,
+                                    nsGkAtoms::dfn,
+                                    nsGkAtoms::em,
+                                    nsGkAtoms::font,
+                                    nsGkAtoms::i,
+                                    nsGkAtoms::ins,
+                                    nsGkAtoms::kbd,
+                                    nsGkAtoms::mark,
+                                    nsGkAtoms::s,
+                                    nsGkAtoms::samp,
+                                    nsGkAtoms::small,
+                                    nsGkAtoms::span,
+                                    nsGkAtoms::strike,
+                                    nsGkAtoms::strong,
+                                    nsGkAtoms::sub,
+                                    nsGkAtoms::sup,
+                                    nsGkAtoms::time,
+                                    nsGkAtoms::tt,
+                                    nsGkAtoms::u,
+                                    nsGkAtoms::var)) {
+    return false;
+  }
+
+  // If the element is unknown element, we shouldn't insert line breaks before
+  // it since unknown elements should be ignored.
+  RefPtr<HTMLUnknownElement> unknownHTMLElement = do_QueryObject(aContent);
+  return !unknownHTMLElement;
 }
 
 nsresult
 ContentEventHandler::GenerateFlatTextContent(nsRange* aRange,
                                              nsAFlatString& aString,
                                              LineBreakType aLineBreakType)
 {
   NS_ASSERTION(aString.IsEmpty(), "aString must be empty string");

--- a/dom/html/HTMLUnknownElement.cpp
+++ b/dom/html/HTMLUnknownElement.cpp
@@ -9,16 +9,19 @@
 #include "mozilla/dom/HTMLElementBinding.h"
 #include "jsapi.h"
 
 NS_IMPL_NS_NEW_HTML_ELEMENT(Unknown)
 
 namespace mozilla {
 namespace dom {
 
+NS_IMPL_ISUPPORTS_INHERITED(HTMLUnknownElement, nsGenericHTMLElement,
+                            HTMLUnknownElement)
+
 JSObject*
 HTMLUnknownElement::WrapNode(JSContext *aCx, JS::Handle<JSObject*> aGivenProto)
 {
   return HTMLUnknownElementBinding::Wrap(aCx, this, aGivenProto);
 }
 
 NS_IMPL_ELEMENT_CLONE(HTMLUnknownElement)
 

--- a/dom/html/HTMLUnknownElement.h
+++ b/dom/html/HTMLUnknownElement.h
@@ -7,29 +7,40 @@
 #define mozilla_dom_HTMLUnknownElement_h
 
 #include "mozilla/Attributes.h"
 #include "nsGenericHTMLElement.h"
 
 namespace mozilla {
 namespace dom {
 
+#define NS_HTMLUNKNOWNELEMENT_IID \
+{ 0xc09e665b, 0x3876, 0x40dd, \
+  { 0x85, 0x28, 0x44, 0xc2, 0x3f, 0xd4, 0x58, 0xf2 } }
+
 class HTMLUnknownElement final : public nsGenericHTMLElement
 {
 public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_HTMLUNKNOWNELEMENT_IID)
+
+  NS_DECL_ISUPPORTS_INHERITED
+
   explicit HTMLUnknownElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo)
     : nsGenericHTMLElement(aNodeInfo)
   {
     if (NodeInfo()->Equals(nsGkAtoms::bdi)) {
       SetHasDirAuto();
     }
   }
 
   virtual nsresult Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const override;
 
 protected:
+  virtual ~HTMLUnknownElement() {}
   virtual JSObject* WrapNode(JSContext *aCx, JS::Handle<JSObject*> aGivenProto) override;
 };
 
+NS_DEFINE_STATIC_IID_ACCESSOR(HTMLUnknownElement, NS_HTMLUNKNOWNELEMENT_IID)
+
 } // namespace dom
 } // namespace mozilla
 
 #endif /* mozilla_dom_HTMLUnknownElement_h */

--- a/widget/tests/window_composition_text_querycontent.xul
+++ b/widget/tests/window_composition_text_querycontent.xul
@@ -3186,16 +3186,29 @@ function runSetSelectionEventTest()
      "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection anchor node should be the text node");
   is(selection.anchorOffset, 0,
      "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection anchor offset should be 0");
   is(selection.focusNode, contenteditable.childNodes.item(1).lastChild,
      "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection focus node should be the text node");
   is(selection.focusOffset, 0,
      "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection focus offset should be 0");
   checkSelection(0, "", "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\"");
+
+  // #16
+  contenteditable.innerHTML = "a<blink>b</blink>c";
+  synthesizeSelectionSet(0, 3);
+  is(selection.anchorNode, contenteditable.firstChild,
+     "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection anchor node should be the first text node");
+  is(selection.anchorOffset, 0,
+     "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection anchor offset should be 0");
+  is(selection.focusNode, contenteditable.lastChild,
+     "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection focus node should be the last text node");
+  is(selection.focusOffset, contenteditable.lastChild.wholeText.length,
+     "runSetSelectionEventTest #15 (0, 0), \"" + contenteditable.innerHTML + "\": selection focus offset should be the length of the last text node");
+  checkSelection(0, "abc", "runSetSelectionEventTest #16 (0, 3), \"" + contenteditable.innerHTML + "\"");
 }
 
 function runQueryTextContentEventTest()
 {
   contenteditable.focus();
 
   var result;
 
@@ -3382,16 +3395,22 @@ function runQueryTextContentEventTest()
   result = synthesizeQueryTextContent(kLFLen, kLFLen * 2);
   is(result.text, kLF + kLF, "runQueryTextContentEventTest #8 (kLFLen, kLFLen*2), \"" + contenteditable.innerHTML + "\"");
 
   result = synthesizeQueryTextContent(kLFLen*2, kLFLen);
   is(result.text, kLF, "runQueryTextContentEventTest #8 (kLFLen*2, kLFLen), \"" + contenteditable.innerHTML + "\"");
 
   result = synthesizeQueryTextContent(kLFLen*3, 1);
   is(result.text, "", "runQueryTextContentEventTest #8 (kLFLen*3, 1), \"" + contenteditable.innerHTML + "\"");
+
+  // #16
+  contenteditable.innerHTML = "a<blink>b</blink>c";
+
+  result = synthesizeQueryTextContent(0, 3);
+  is(result.text, "abc", "runQueryTextContentEventTest #16 (0, 3), \"" + contenteditable.innerHTML + "\"");
 }
 
 function runCSSTransformTest()
 {
   textarea.focus();
   textarea.value = "some text";
   textarea.selectionStart = textarea.selectionEnd = textarea.value.length;
   var editorRect = synthesizeQueryEditorRect();